Air Force Memorial a Tribute to Flight and Engineering

Illustration of the Air Force Memorial by Arup's 3D Media Group - For The Washington Post (Illustration by Arup's 3D Media Group - For The Washington Post)

But wind happens. So while the spires are anchored firmly to a buried concrete pedestal, their upper portions are sure to sway. Getting that elasticity just right was a big part of Freed's challenge.

That challenge was in part aesthetic. Although some motion was inevitable, "we didn't want it to look like tall reeds in the wind," said Michael D. Flynn of Pei Cobb.

But of equal concern was the monument's structural integrity. Wind can transfer large amounts of energy to a structure, explained Peter A. Irwin, president of Rowan Williams Davies and Irwin Inc. of Guelph, Ontario, the consulting firm that put a one-fortieth-scale model of the memorial through its paces in a six-foot-tall wind tunnel.

"Lightweight steel structures have very little ability to dissipate that energy," Irwin said. Over time, that can make a structure oscillate.

"It's rather like pushing a child on a swing," Irwin said. "Oscillation will just grow."

Grow, that is, until the structure finally gallops beyond its elastic capacity and crashes to the ground -- or into the waters of Puget Sound, as famously happened to an undulating, imperfectly engineered Tacoma Narrows Bridge in Washington state on a windy day in November 1940.

This is where the art and science of "damping" comes in -- the process of intentionally dissipating the energy in a structure.

High-rise buildings often have specialized damping mechanisms, said Leo Argiris, a principal in the New York office of the design engineering firm Arup and project director for the Air Force memorial.

Some buildings use the sway of huge pendulums to scatter accumulated energy. But a pendulum cannot hang in a banana-shaped structure.

Other structures sport roughened outer surfaces, indentations or clipped corners that, in effect, "confuse the wind," Argiris said. But that approach would have seriously compromised the appearance of the spires, which have been specially blasted with glass beads to get a velvety metallic finish reminiscent of aircraft skin.

Balls in boxes offered a solution. The approach is rarely considered in larger structures -- enormous spheres would be needed to stabilize an entire building -- but for the spires, they were perfect.

Each steel box is double-walled, and the two-inch space between those walls is filled with Sorbothane, an elastic energy-absorbing polymer commonly found in the insoles of shoes. The balls are lead, with an outer coating of stainless steel. When wind sets the mid- and upper reaches of the spires swaying, the balls roll about, banging into the walls of their padded cells.

Computer simulations predict that those random impacts will redistribute enough of the energy imparted by the wind to prevent worrisome oscillations from building up -- ultimately dissipating that energy as imperceptible wavelets of heat.

One box assembly was road-tested on a large "shaker table" designed to simulate earthquakes. That is when it became clear that metal balls in metal boxes do not give up all their energy as heat. Some of it merges as clanging.

No one knows whether the rattling of those 13 balls in their cages will be perceptible to the public in a stiff breeze. Given all the muffling concrete and the noise of traffic on nearby Interstate 395, it seems unlikely, said retired Maj. Gen. Ed Grillo, president of the Air Force Memorial Foundation, which has ushered the project through more than a decade of planning.

But for those who are aware of the stealthy stabilizers, the fact that they are in there, out of public view yet preserving and protecting, may seem apt.

"This memorial will mean many things to many people," Grillo said. "I certainly hope it will serve as an inspiration to future generations to serve their country."

If not in the Air Force, then perhaps as architects and engineers.